Energy storage in the primary photochemical events of rhodopsin and isorhodopsin

Abstract

The energetics associated with the photoequilibrium rhodospin (R) .**GRAPHIC**. bathorhodopsin (B) .**GRAPHIC**. isorhodopsin (I) are measured at 77 K by using pulsed-laser photocalorimetry and a range of excitation wavelengths and relative starting concentrations. Enthalpies for the photochemical transformations R .**GRAPHIC**. B and I .**GRAPHIC**. B are measured to be .DELTA.HRB = 32.2 .+-. 0.9 kcal mol-1 and .DELTA.HIB = 27.1 .+-. 3.2 kcal mol-1, respectively. Although the value of .DELTA.HRB is slightly lower than that reported previously by Copper of 34.7 .+-. 2.2 kcal mol-1 [Copper, A. (1979) Nature (London) 282, 531-533], the two values are in agreement within experimental error. The energy difference .DELTA.HRB - .DELTA.HIB = 5.1 .+-. 3.3 kcal mol-1 is identical within experimental error with the difference in enthalpies of isorhodopsin and rhodopsin [5.2 .+-. 2.3; Copper, A. (1979) FEBS Lett. 100, 382-384]. We suggest that this result is consistent with the theory that bathorhodopsin is a single, common photochemical intermediate connecting rhodopsin and isorhodopsin.